Biological classification

    Cards (70)

    • Since the dawn of civilisation, there have been many attempts to classify living organisms
    • Aristotle was the earliest to attempt a more scientific basis for classification
    • Aristotle's classification
      • Used simple morphological characters to classify plants into trees, shrubs and herbs
      • Divided animals into two groups, those which had red blood and those that did not
    • In Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed
    • The two kingdom classification was found inadequate as a large number of organisms did not fall into either category
    • Besides gross morphology, a need was also felt for including other characteristics like cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, evolutionary relationships, etc.
    • Classification systems for the living organisms have undergone several changes over time
    • Though plant and animal kingdoms have been a constant under all different systems, the understanding of what groups/organisms be included under these kingdoms have been changing
    • The number and nature of other kingdoms have also been understood differently by different scientists over time
    • Five Kingdom Classification
      Kingdoms defined by R.H. Whittaker in 1969: Monera, Protista, Fungi, Plantae and Animalia
    • Criteria used by Whittaker for classification
      • Cell structure
      • Body organisation
      • Mode of nutrition
      • Reproduction
      • Phylogenetic relationships
    • The three-domain system has also been proposed that divides the Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain and thereby a six kingdom classification
    • Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and the angiosperms under 'Plants'
    • The character that unified the 'Plant' kingdom was that all the organisms included had a cell wall in their cells
    • This placed together groups which widely differed in other characteristics, like the prokaryotic bacteria and the blue green algae with the eukaryotic and multicellular organisms
    • The classification did not differentiate between the heterotrophic group - fungi, and the autotrophic green plants, though they also showed a characteristic difference in their wall composition
    • When such characteristics were considered, the fungi were placed in a separate kingdom - Kingdom Fungi
    • All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista
    • Kingdom Protista has brought together organisms which, in earlier classifications, were placed in different kingdoms
    • This happened because the criteria for classification changed
    • Over time, an attempt has been made to evolve a classification system which reflects not only the morphological, physiological and reproductive similarities, but is also phylogenetic, i.e., is based on evolutionary relationships
    • Shapes of bacteria
      • Cocci (spherical)
      • Bacilli (rod-shaped)
      • Vibrio (comma-shaped)
      • Spirilla (spiral)
    • Bacteria
      • They are the most abundant micro-organisms
      • They occur almost everywhere, including extreme habitats
      • Many live in or on other organisms as parasites
    • Autotrophic bacteria
      They synthesise their own food from inorganic substrates, either photosynthetically or chemosynthetically
    • Heterotrophic bacteria
      They depend on other organisms or on dead organic matter for food
    • Archaebacteria
      • They live in harsh habitats like extreme salty areas, hot springs and marshy areas
      • They have a different cell wall structure which helps them survive in extreme conditions
      • Methanogens in the gut of ruminant animals produce methane (biogas)
    • Eubacteria
      • They have a rigid cell wall and, if motile, a flagellum
      • Cyanobacteria are photosynthetic autotrophs
      • Chemosynthetic autotrophic bacteria oxidise inorganic substances and use the released energy for ATP production
      • Heterotrophic bacteria are important decomposers, some are pathogens causing diseases
    • Bacterial reproduction
      1. Fission
      2. Spore formation
      3. Primitive type of DNA transfer (sexual reproduction)
    • Mycoplasma
      Organisms that completely lack a cell wall, the smallest living cells known, can survive without oxygen, many are pathogenic
    • Members of Protista are primarily aquatic
    • Protista
      • They are single-celled eukaryotes
      • Their cell body contains a well defined nucleus and other membrane-bound organelles
      • Some have flagella or cilia
      • They reproduce asexually and sexually by a process involving cell fusion and zygote formation
    • Groups included under Protista
      • Chrysophytes
      • Dinoflagellates
      • Euglenoids
      • Slime moulds
      • Protozoans
    • Chrysophytes
      • They include diatoms and golden algae
      • They are microscopic and float passively in water currents (plankton)
      • Most are photosynthetic
      • Diatoms have cell walls embedded with silica, forming two overlapping shells
    • Protista
      A kingdom that includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans
    • Protista
      • Primarily aquatic
      • Eukaryotic with well-defined nucleus and membrane-bound organelles
      • Some have flagella or cilia
      • Reproduce asexually and sexually
    • Chrysophytes
      A group that includes diatoms and golden algae
    • Chrysophytes
      • Diatoms
      • Golden algae (desmids)
    • Chrysophytes
      • Found in fresh water and marine environments
      • Microscopic and float passively in water currents (plankton)
      • Mostly photosynthetic
      • Diatoms have silica cell walls that accumulate as 'diatomaceous earth'
      • Diatoms are chief producers in oceans
    • Dinoflagellates
      Mostly marine and photosynthetic organisms
    • Dinoflagellates
      • Cell wall has stiff cellulose plates
      • Have two flagella, one longitudinal and one transverse
      • Red dinoflagellates (e.g. Gonyaulax) can cause 'red tides' and release toxins
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